1. Field of the Invention
The present invention relates to a thin film magnetic head. Specifically, the present invention relates to the device structure of a thin film magnetic head equipped with a pair of magnetic layers that change in the direction of magnetization in response to an external magnetic field.
2. Description of the Related Art
As the high-density recording of hard disk drive (HDD) has been developed, heads of high sensitivity and high output have been required. A spin valve head was invented as a head that satisfies the aforementioned requirements. A spin valve head is provided with a pair of ferromagnetic layers on both sides of a nonmagnetic middle layer. An antiferromagnetic layer is disposed in contact with one ferromagnetic layer, and the direction of magnetization in the ferromagnetic layer is fixed in one direction by means of exchange coupling with the antiferromagnetic layer. The direction of magnetization in the other ferromagnetic layer freely rotates in response to an external magnetic field. Such a ferromagnetic layer is referred to as a free layer. In a spin valve head, the change in the relative angle of spins in these two ferromagnetic layers brings about the change in magnetic resistance. A pair of the aforementioned ferromagnetic layers is put between a pair of shield layers, whereby an external magnetic field applied from an adjacent bit on the same track of a recording medium is shielded.
The exchange coupling between an antiferromagnetic layer and a ferromagnetic layer is one of essential characteristics in a spin valve head. Nevertheless, as high-density recording is further advanced, an antiferromagnetic layer cannot be contained within a read gap (i.e., the width of a medium signal in the traveling direction in the medium at a time when a magnetic head reads the signal, which is correlated with the thickness of a film put between shields) if the read gap approaches about 20 nm. Accordingly, we need technology for changing the relative angle between the directions of magnetization in two ferromagnetic layers in response to an external magnetic field by controlling the direction of magnetization in a ferromagnetic layer by some means. U.S. Pat. No. 7,035,062 has disclosed a thin film magnetic head comprising two free layers that change in the direction of magnetization in response to an external magnetic field and a nonmagnetic middle layer put between the aforementioned two free layers. The two free layers are exchange-coupled via the nonmagnetic middle layer by the RKKY (Rudermann, Kittel, Kasuya, Yoshida) interaction and are magnetized in the antiparallel direction to each other at a time when there is no magnetic field applied (i.e., the magnetic field-free state, as used herein). On the rear face of the two free layers and nonmagnetic middle layer seen from the opposing face of a recording medium (or an air bearing surface, hereinafter ABS) is provided a bias magnetic layer, and the bias magnetic field is applied in the direction at right angles to the ABS.
A magnetic field applied from the bias magnetic layer causes the direction of magnetization to have a specific relative angle in the two free layers. If an external magnetic field is applied from a recording medium in the direction at right angles to the ABS at the time, the relative angle between the directions of magnetization in the two free layers will be changed, which leads to the change in the electric resistance of a sense current. As a result, the external magnetic field can be detected. Thus, using two free layers in the film configuration eliminates an antiferromagnetic layer, thereby simplifying the film configuration and easily reducing a read gap. As used herein, the term “parallel” refer to the state in which the direction of magnetization is parallel to each other in the same direction and the term “antiparallel” refers to the state in which the direction of magnetization is parallel to each other in the opposite direction.
Nevertheless, in the thin film magnetic head using a method for magnetically connecting two free layers by the RKKY interaction, the type of materials usable as a nonmagnetic middle layer is limited. Nor can any improvement in the rate of change in magnetoresistance be expected. Accordingly, another technology is required for magnetizing two free layers in the antiparallel direction to each other.